<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .container {
            display: flex;
            width: 1000px;
            height: 200px;
            border: solid 1px red;
        }

        .container .item {
            border: solid 1px;
            flex-basis: auto;
            width: 600px;
        }

        .container .item1 {
            /*flex-basis: 4000px;*/
            /*width: 4000px;*/
        }

        .container .item2 {
            /*width: 100px;*/
        }

        .container .item3 {
            /*flex-basis: 100px;*/
        }

        .container .item4 {
            /*width: 100px;*/
        }
    </style>
</head>
<body>
<div class="container">
    <div class="item item1">
        flexBasis
    </div>
<!--
    <div class="item item1">
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
        flexBasis/flexBasis/flexBasis/flexBasis/flexBasis/
    </div>
-->
    <div class="item item2">width</div>
    <div class="item item3">flexBasis</div>
    <div class="item item4">width</div>
</div>
</body>
</html>